You most likely would need to show the BACKS of the cards. These cards have been faked so many times.
Theres only 1 true backing on these cards. As a Mantle collector, I was lucky enough people gave me this info:
the light tanish color is the original. Not go buy the grey or white paper stock backs=FAKE

Here is a photo of the backs along with a commonly seen forgery/fake. They are quite easy to identify from the back, and if you have been around them enough you can see the difference on the fronts as well.